How to create a DLL callable from batch using rundll32.exe

Question

With rundll32 it is possible to call simple APIs into DLLs.

Two questions:

• for this usage, should be the entry point use the __cdecl calling convention, or some other calling convention?
• is it possible to pass only numeric args or also strings?

Answer 1

Don't use rundll32.exe. I can't word it any better than Raymond Chen did: What’s the guidance on when to use rundll32? Easy: Don’t use it.

If you read through the blog entry, and are still convinced, that this is the solution you need, here is the deal: The entry point must follow the __stdcall calling convention:

void CALLBACK
EntryPointW(HWND hwnd, HINSTANCE hinst, LPWSTR lpszCmdLine, int nCmdShow);

The trailing W is important, so that the lpszCmdLine argument is passed as Unicode (UTF-16LE). EntryPoint is a placeholder, it can be any legal symbol.

As an example, you can export a symbol called MyFunctionW, and pass an arbitrary command line using:

rundll32.exe MyDll.dll,MyFunction 132 C:\WINDOWS\INF\SHELL.INF

But really, you should evaluate other options (e.g. PowerShell). Convenience doesn't come for free.

Answer 2

The entry point must use the __stdcall calling convention:

  void CALLBACK
  EntryPoint(HWND hwnd, HINSTANCE hinst, LPSTR lpszCmdLine, int nCmdShow);

This is covered in the following documentation:

NFO: Windows Rundll and Rundll32 Interface